Monday, September 17, 2012

Felt Intensity – What You Feel is What You Get!

How do we size an earthquake?

A few earthquakes over the last few months have got me
thinking about how we talk about earthquake size. In the western world we
are quite focused on magnitude, but that only gives a starting point. I have
already bored you enough about earthquake magnitude in a few previous blogs
(see What’s
the Magnitude?; Deep
Earthquakes and Magnitudes; and Deep
Earthquakes and Magnitude – Again!) – but to recap, an earthquake magnitude
is an estimate of the true size of an earthquake independent of the observer
(or where the observer is). The magnitude is only the start of the story if you
want to understand the likely impacts of an earthquake. Not all earthquakes are
created equal; some are more energetic than others even if they have the same
magnitude. Some direct shaking energy towards where people live, and where you
are compared to the earthquake source is very important. All this leads to the
idea of shaking intensity – a mapping of the levels of shaking caused by an
earthquake rather than a single number like magnitude.

Modified Mercalli Shaking Intensity

If you want to characterise how you feel an earthquake, then
felt intensity is what you are after. This is a measure of the shaking where
you are, and is given (at least in New Zealand) by the Modified
Mercalli (MM) scale which covers the range from not felt (MM 1:
Imperceptible) to complete destruction (MM 12: Completely devastating).
Obviously an earthquake’s impact and the level of damage it causes are related
to the intensity. The MM value at a particular place depends on the distance
from the earthquake, its size and depth, the kind of rocks between you and the
source and material you or the building you are in rests on.

ShakeMap (or where did the map go?)

On the old GeoNet website we had a display on the front page
based on the shaking at each recording station (Figure 1). Although this gave a
good indication of the where maximum shaking levels were being recorded by our
instruments (and I know some of you want it back!), it could be biased by
instrument issues and was often misinterpreted. So it was good for a quick
look, but not really very useful for characterising the potential damage in
detail.

Figure 1: The "ShakeNZ" plot for the Christchurch Earthquake of 22 February 2011. This map shows the shaking levels as squares around the sensor sites which change colour and get larger as the shaking level increases.

We are working towards having a ShakeMap (as developed by
the United States Geological Survey, see the USGS Shakemap site)
available for larger earthquakes which will indicate the distribution of shaking
(see Figure 2 for an example). This map will be produced within a few minutes
of an earthquake occurring and be based on data from the sensor sites and a
knowledge of how the earthquake waves travel through the Earth (tailored for
New Zealand conditions). The map will show MM intensity but we will also be
able to provide information in forms that are suitable for use by engineers
interested in the level of shaking experienced by buildings or other structures
in the region. This can include shaking accelerations at different periods of
oscillation – different size structures are more susceptible to different
shaking oscillations caused by earthquakes.

Current planning should see ShakeMap on the new GeoNet website
within the next few months.

Figure 2: An example ShakeMap for the Christchurch Earthquake of 22 February 2011. This is an example of what the ShakeMap on the new GeoNet website may look like.

So what about shaking duration?

This is a hard one as the perceived duration will depend
both on the size of the earthquake and where you are (a bit like intensity) but
is also very dependent on the near surface structure under your feet. For
example, if you live in a valley the shaking waves will “bounce around” in the
valley and the shaking will go on for much longer than if you were on a hard
rock site. We can estimate how long the fault takes to rupture (by studying the
earthquake waves recorded on our instruments), but how long the Earth shakes
depends on the size and distance, and how many ways the earthquake waves reach
you (some waves “bounce” around in the Earth so the shaking goes on much longer
than the fault break time). For these reasons we do not usually use duration as
a measure of earthquake size.

To put this in terms of recent experience the fault-break of
the Christchurch Earthquake (22 February 2011) was over in just a few seconds,
but the shaking went on longer because of the near-surface structure under the
city. But the total duration in areas of maximum damage was only around 10 - 15
seconds. Compare that to a possible Alpine Fault earthquake much further away
from Christchurch where the shaking intensity would be much less in the city (in
fact even much less than the Darfield Earthquake of 4 September 2010) but the
shaking would go on for minutes. Duration is not a good indication of likely earthquake impacts.

Christopher, that map is no longer on the new site but we will be adding more maps as time goes on. Making such a large change so we could bring fast earthquake locations to the website resulted in some compromises (at least in the short term). Did you find that map useful?

About Me

I have been the Director of New Zealand GeoNet at GNS Science since 2005. GeoNet is New Zealand’s integrated geological hazards monitor system employing state of the art equipment and telecommunications technology. I am the immediate past chair of the governance group for the Pacific Tsunami Warning and Mitigation System (ICG/PTWS), and a current working group chair.. I am a scientific project manager, seismologist, scientific instrumentation and telecommunications specialist with more than 30 years’ experience. My research has concentrated on geophysical instrumentation, the field studies of large earthquakes, and the study of the deep structure beneath New Zealand and internationally using the seismic waves generated by earthquakes.